1. Field of the Invention
The present invention relates to the preparation of a zeolite film on a substrate, and more particularly to the preparation of a continuous and transparent zeolite film having substantial preferred crystallographic orientation on a substrate.
2. Description of the Related Art
Zeolite, which is a member of a family of minerals known as tectosilicates, is microporous crystalline material with high internal surface area formed by three-dimensional open framework of tetrahedral SiO4 units connected by oxygen atoms thereof. Aluminum or other atoms, leading to negative charges that must be neutralized by cations, may replace the silicon atoms in the zeolite framework. Tremendous zeolite exists in nature whereas synthetic zeolite, not being found in nature, has been prepared and is commercially available.
Although the three-dimensional framework structure of zeolite, as well as the resulted pore size and volume of crystal structure thereof, differs from case to case, the pore size in general falls in the range of 0.3 to 1.0 nm. Due to the molecular sized pore structure, zeolite is frequently used as molecular selective catalysts and adsorbent. In such applications, zeolite is usually employed in the form of powder, pellet and even other particle forms.
Zeolite in the form as polycrystalline film on a porous substrate has been disclosed for various membrane applications. For embodiment, U.S. Pat. No. 4,699,892 (1987) describes the preparation of a permeable composite zeolite membrane for the selectively separation of materials. U.S. Pat. No. 5,258,339 (1993) describes a method of forming a zeolite membrane by contacting a porous support with a sol composition, and hydrothermally converting the composition into a zeolite membrane. A long list of other patens has been published on a variety of improvements for the preparation of supported zeolite membrane. (U.S. Pat. No. 5,266,542 (1993), U.S. Pat. No. 5,464,798, U.S. Pat. No. 5,429,743 (1995), U.S. Pat. No. 5,567,664 (1996), U.S. Pat. No. 5,605,631 (1997), U.S. Pat. No. 5,895,769, U.S. Pat. No. 5,871,650 (1999), U.S. Pat. No. 6,051,517, U.S. Pat. No. 6,140,263, U.S. Pat. No. 6,159,542 (2000), U.S. Pat. No. 6,193,784 and U.S. Pat. No. 6,197,427 (2001))
In addition to zeolite membrane, a zeolite film coated on a non-porous substrate also finds applications. For embodiment, U.S. Pat. No. 5,151,110 zeolite was coated on piezoelectric substrate for nanogram level detection of chemicals. U.S. Pat. No. 5,841,021 dispersed zeolite in a nonporous polymer membrane coated on electrode for sensor application. U.S. Pat. No. 5,843,392 described the coating of substantially orientated zeolite crystals on structured support for catalytic purpose. US patent application 20010008662 described the combination of oxygen scavenger layer and a zeolite layer as an active oxygen barrier packaging materials. US patent application 20020110699 disclosed that zeolite coating on metal surface functions as protection against corrosion. For applications such as protective against corrosion or as gas barrier, the zeolite coating must be continuous and without inter-crystalline void.
Other potential applications for zeolite coating ranged from molecular selective sensor, electrode, as low refractive index layer, electro-optical component, low κ dielectric film (Wang et al., 2001) and anti-microbial coating (Cowan et al. 2003) for metals. The most interesting possible applications may be in the electro-optical field. The potential of using zeolite in optics was demonstrated more than a decade ago by Ozin (1989), who was able to prepare CdS clusters in the ordered cavities of Y zeolite single crystal to form aligned quantum dots. Marlow et al. (1994, 1996) later demonstrated that dipolar chromophores (p-nitroanilin) adsorbed in large AlPO4-5 crystals aligned into long dipole chain and acted as second harmonic generator. Following the same direction, Calzaferri et al. (2001, 2003) loaded zeolite L with dye to prepare an optical antenna. The entrapment of guest molecules in cages of zeolite has attracted a great deal of attentions for its possibility in manipulating both the optical properties of the guest molecule and host structure. However, to realize the potential opto-electronic applications, the zeolite framework must first be grown to a manageable size or coated on substrate as macroscopic oriented film. It is further desirable that the so coated film is transparent. To capitalize on the possibility of aligned dipoles, a preferred orientation of the crystallographic plans is needed.
There are roughly three general procedures known for the preparation. A typical way is called in-situ hydrothermal crystallization. This comprises of immersing the substrate in a synthesis solution, and heating the substrate and solution to high temperature under autogeneous pressure. During the process, heterogeneous nucleation of zeolite occurs on the substrate surface, from which zeolite film is grown. Such in-situ hydrothermal crystallization approach was claimed in U.S. Pat. No. 4,699,892 for the preparation of zeolite membrane, and was described in US patent application No. 20020134995 for the preparation of low dielectric layer on silicon wafer. However, the zeolite film obtained in this approach is usually relative thick and without preferential orientation, and its inter-crystalline void is difficult to eliminate.
Another approach as described by U.S. Pat. No. 5,258,339 is to coat a porous substrate with fresh zeolite synthesis sol that will gel before fully penetrating the porous substrate, and treat the coated substrate under saturated water vapor to convert the gel to zeolite. Again, the zeolite coating produced is relative thick and without preferential orientation.
Yet another approach, as disclosed by International Applications PCT/EP93101209, and PCT/US95/08514 published as WO 96/01687, is the deposition of previously formed zeolite micro- or nanocrystals as nucleation layers on substrates before the in-situ hydrothermal process. The zeolite film obtained shows some preferred orientation. To improve the orientation, U.S. Pat. No. 6,177,373 describe the preparation of a dispersion comprising discrete microcrystals of zeolite which have a surface charge. The charged microcrystals are then attached to a substrate with opposite surface charge to form a monolayer. The substrate comprising a monolayer of zeolite is then calcined in steam to a temperature between 200 to 1000.° C. to fix said monolayer in place. Due to the monolayer attachment of microcrystals on the substrate, they are oriented in similar direction. However, these procedures are complicate since it involves the preparation and separation of microcrystals in the first step, and the coating and calcination in a second step.